docs(repo): add PRP-33 batch-runner-mvp (#277)

[w7-mgfcode]

Summary

Lands the PRP doc for the Batch Runner MVP — the unblocker for the four downstream Full-Version INITIALs (parallel-execution, priority-queue, export-and-retry, champion-and-heatmap) merged via #278.

Doc-only PR. The implementation lands in a follow-up execution PR.

Refs #277.

Drift edits applied vs INITIAL-batch-runner-mvp.md

The PRP draft was reviewed against its source INITIAL (PRPs/INITIAL/INITIAL-batch-runner-mvp.md) and found to be MINOR DRIFT. Three edits applied before landing:

1. Locked sample_size derivation to Option (a) — computed inside the batch slice from fold_metrics. Option (b) (extending jobs/service.py:_shape_backtest_result) is now explicitly REJECTED in the Confidence-Score section because it would violate the vertical-slice no-cross-import rule and the off-limits boundary declared in § "What → Success Criteria".
2. Added test_service_emits_lifecycle_events to the Test Plan unit suite — asserts the five batch.{created,item_started,item_completed,item_failed,completed} structlog event names emit in order and that every event payload carries a request_id field for log correlation.
3. Rounded out _expand_scope unit coverage — added explicit test entries for region, category, and all BatchScope.kind values (PRP previously only enumerated manual and top_revenue).

Out of scope for this PR

• No code under app/features/batch/, no Alembic migration, no frontend code — those land in the implementation PR per PRP execution.
• No changes to app/features/jobs/, app/features/forecasting/, or tests/test_e2e_demo.py — the PRP declares these off-limits.

Test plan

• PRP file renders correctly (review the diff for the three edit spots).
• Commit message accepted by .claude/hooks/check-commit-format.sh (uses repo scope; already in allow-list).
• Reviewer verifies the three drift edits read as intended.

Note: .claude/rules/commit-format.md is gitignored, so the PRP's "Task 1 — Pre-add batch to commit-format scope allow-list" step runs locally on whichever workstation executes the implementation PRP; it cannot ride along in this PR.

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